Earth is bombarded by meteors, occasionally by one large enough to cause a significant explosion and possible loss of life. It is not possible to detect all hazardous asteroids, and the efforts to ...detect them years before they strike are only advancing slowly. Similarly, ideas for mitigation of the danger from an impact by moving the asteroid are in their infancy. Although the odds of a deadly asteroid strike in the next century are low, the most likely impact is by a relatively small asteroid, and we suggest that the best mitigation strategy in the near term is simply to move people out of the way. With enough warning, a small asteroid impact should not cause loss of life, and even portable property might be preserved. We describe an early warning system that could provide a week’s notice of most sizeable asteroids or comets on track to hit the Earth. This may be all the mitigation needed or desired for small asteroids, and it can be implemented immediately for relatively low cost. This system, dubbed Asteroid Terrestrial-Impact Last Alert System (ATLAS), comprises two observatories separated by about 100 km that simultaneously scan the visible sky twice a night. Software automatically registers a comparison with the unchanging sky and identifies everything that has moved or changed. Communications between the observatories lock down the orbits of anything approaching the Earth, within one night if its arrival is less than a week. The sensitivity of the system permits detection of 140 m asteroids (100 Mton impact energy) three weeks before impact and 50 m asteroids a week before arrival. An ATLAS alarm, augmented by other observations, should result in a determination of impact location and time that is accurate to a few kilometers and a few seconds. In addition to detecting and warning of approaching asteroids, ATLAS will continuously monitor the changing universe around us: most of the variable stars in our Galaxy, many microlensing events from stellar alignments, luminous stars and novae in nearby galaxies, thousands of supernovae, nearly a million quasars and active galactic nuclei, tens of millions of galaxies, and a billion stars. With two views per day ATLAS will make the variable universe as familiar to us as the sunrise and sunset.
Abstract
We present a new 3D map of interstellar dust reddening, covering three quarters of the sky (declinations of δ ≳ −30°) out to a distance of several kiloparsecs. The map is based on ...high-quality stellar photometry of 800 million stars from Pan-STARRS 1 and 2MASS. We divide the sky into sightlines containing a few hundred stars each, and then infer stellar distances and types, along with the line-of-sight dust distribution. Our new map incorporates a more accurate average extinction law and an additional 1.5 yr of Pan-STARRS 1 data, tracing dust to greater extinctions and at higher angular resolutions than our previous map. Out of the plane of the Galaxy, our map agrees well with 2D reddening maps derived from far-infrared dust emission. After accounting for a 25 per cent difference in scale, we find a mean scatter of ∼10 per cent between our map and the Planck far-infrared emission-based dust map, out to a depth of 0.8 mag in E(gP1 − rP1), with the level of agreement varying over the sky. Our map can be downloaded at http://argonaut.skymaps.info, or from the Harvard Dataverse (Green 2017).
We present an algorithm to identify the type of an SN spectrum and to determine its redshift and age. This algorithm, based on the correlation techniques of Tonry & Davis, is implemented in the ...Supernova Identification (SNID) code. It is used by members of ongoing high-redshift SN searches to distinguish between Type Ia and Ib/c SNe and to identify "peculiar" SNe Ia. We develop a diagnostic to quantify the quality of a correlation between the input and template spectra, which enables a formal evaluation of the associated redshift error. Furthermore, by comparing the correlation redshifts obtained using SNID with those determined from narrow lines in the SN host galaxy spectrum, we show that accurate redshifts (with a typical error sigma sub(z) unk 0.01) can be determined for SNe Ia without a spectrum of the host galaxy. Last, the age of an input spectrum is determined with a typical accuracy sigma sub(i) unk 3 days, shown here by using high-redshift SNe Ia with well-sampled light curves. The success of the correlation technique confirms the similarity of some SNe Ia at low and high redshifts. The SNID code, which will be made available to the community, can also be used for comparative studies of SN spectra, as well as comparisons between data and models.
The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W (-SDSS g) and F850LP (-SDSS z) filters. We derive distances for 84 of ...these galaxies using the method of surface brightness fluctuations (SBFs), present the SBF distance catalog, and use this database to examine the three-dimensional distribution of early-type galaxies in the Virgo Cluster. The SBF distance moduli have a mean (random) measurement error of 0.07 mag (0.5 Mpc), or roughly 3 times better than previous SBF measurements for Virgo Cluster galaxies. Five galaxies lie at a distance of d - 23 Mpc and are members of the W' cloud. The remaining 79 galaxies have a narrow distribution around our adopted distance of < d > = 16.5 c0.1 (random mean error) c1.1 Mpc (systematic). The rms distance scatter of this sample is s(d) = 0.6 c0.1 Mpc, with little or no dependence on morphological type or luminosity class (i.e., 0.7 c0.1 and 0.5 c0.1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of early-type galaxies is 2.4 c 0.4 Mpc (i.e., c2 s of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16.7 c 0.2 and 16.4 c 0.2 Mpc, respectively. There may be a third subcluster associated with M86. A weak correlation between velocity and line-of-sight distance may be a faint echo of the cluster velocity distribution not having yet completely virialized. In three dimensions, Virgo's early-type galaxies appear to define a slightly triaxial distribution, with axis ratios of (1: 0.7: 0.5). The principal axis of the best-fit ellipsoid is inclined 620-40 from the line of sight, while the galaxies belonging to the W' cloud lie on an axis inclined by 610-15.
We present (g 475 - z 850) color and z 850-band surface brightness fluctuations (SBFs) measurements for 43 early-type galaxies in the Fornax cluster imaged with the Hubble Space Telescope Advanced ...Camera for Surveys. These are combined with our earlier measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z 850-band SBF absolute magnitude as a function of (g 475 - z 850) color, valid for the AB color range 0.8 < (g 475 - z 850) < 1.6. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W' group, and NGC 4697 in the Virgo Southern Extension. The calibration procedure yields a highly precise relative distance modulus for Fornax with respect to Virgo of (m - M) FV = 0.42 ± 0.03 mag, or a distance ratio dF /dV = 1.214 ± 0.017. The resulting Fornax distance modulus is (m - M)For = 31.51 ± 0.03 ± 0.15 mag, corresponding to dF = 20.0 ± 0.3 ± 1.4 Mpc, where the second set of error bars reflects the total systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax cluster galaxies is s d = 0.49+0.11 -0.15 Mpc, or a total line-of-sight depth of 2.0+0.4 -0.6 Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or 'cosmic,' scatter scos in the SBF calibration, unlike the case for the larger Virgo cluster. As a result, we are able to place the first tight constraints on the value of scos. We find scos = 0.06 ± 0.01 mag, with a firm upper limit of scos < 0.08 mag, for the subsample of galaxies with (g 475 - z 850)>1.02, but it is about twice as large for bluer galaxies. We also present an alternative SBF calibration based on the 'fluctuation count' parameter , a proxy for galaxy mass. This gives a consistent relative distance but with larger intrinsic scatter, and we adopt the result from the calibration on (g 475 - z 850) because of its basis in stellar population properties alone. Finally, we find no evidence for systematic trends of the galaxy distances with position or velocity (e.g., no current infall); the Fornax cluster appears both compact and well virialized.
We present a detailed analysis of the morphology, isophotal parameters, and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (M sub(B) = -15.1 mag) to giants ...(M sub(B) = -21.8 mag), imaged in the g and z passbands using the Advanced Camera for Surveys on board the Hubble Space Telescope, Dust and complex morphological structures are common. Dust is detected in 42% of galaxies brighter than B sub(T) = 12.15 mag, while kiloparsec-scale stellar disk, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity. Isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution of 7 pc. For most galaxies, the surface brightness profiles are well described by a Sersic model with index n that increases steadily with the galaxy luminosity; only for 8 of the 10 brightest galaxies are the inner profiles (typically within 100 pc of the center) lower than expected based on an extrapolation of the outer Sersic model, and are better described by a single power-law function. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the logarithmic slope of the inner surface brightness profile, 7; in particular the 7 distribution for galaxies that do not show evidence of multiple morphological components is unimodal across the entire magnitude range spanned by the ACSVCS galaxies. Although the brightest galaxies have shallow inner profiles, the shallowest profiles are found in faint dwarf systems. The widely adopted separation of early-type galaxies between "core" and "power-law" types is questioned based on the present study.
The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging in widely separated bandpasses (F475W - g and F850LP - z) for 100 early-type members of the Virgo ...Cluster, spanning a range of -460 in blue luminosity. We use this large, homogenous data set to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M sub(B) - -15 (66% f sub(n) 82%) and show that ground-based surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated toward the center of Virgo than their nonnucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at M sub(B) 6 -17.6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at M sub(B) 6 -20.5, in the sense that the brighter, "core-Sersic" galaxies lack resolved (stellar) nuclei. A search for nuclei that may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 0."5, all of which are in dwarf galaxies. In each case, however, the evidence suggests that these "nuclei" are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of < r sub(h) > = 4.2 pc, with the sizes of individual nuclei ranging from 62 pc down to ,2 pc (i.e., unresolved in our images) in about a half-dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation r sub(h) 8 super(0.50c0.03). Because the large majority of nuclei are resolved, we can rule out low-level AGNs as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are -3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate age stellar populations. The colors of the nuclei in galaxies fainter than M sub(B) - -17.6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity, and overall frequency. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy properties is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-to-galaxy luminosity ratio in our nucleated galaxies, = -2.49 c 0.09 dex (s = 0.59 c 0.10), is indistinguishable from that of the SBH-to-bulge mass ratio, <log(M sub(; )/M sub(gal))> = -2.61 c 0.07 dex (s = 0.45 c 0.09), calculated in 23 early-type galaxies with detected supermassive black holes (SBHs). We argue that the compact stellar nuclei found in many of our program galaxies are the low-mass counterparts of the SBHs detected in the bright galaxies. If this interpretation is correct, then one should think in terms of "central massive objects"-- either SBHs or compact stellar nuclei--that accompany the formation of almost all early-type galaxies and contain a mean fraction -0.3% of the total bulge mass. In this view, SBHs would be the dominant formation mode above M sub(B) - -20.5.
We characterize the spatial density of the Pan-STARRS1 (PS1) sample of Rrab stars to study the properties of the old Galactic stellar halo. This sample, containing 44,403 sources, spans ...galactocentric radii of 0.55 kpc ≤ Rgc ≤ 141 kpc with a distance precision of 3% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disk and bulge, and halo globular clusters, the sample contains ∼11,000 sources within 20 kpc ≤ Rgc ≤ 131 kpc. We then apply forward modeling using Galactic halo profile models with a sample selection function. Specifically, we use ellipsoidal stellar density models (l, b, Rgc) with a constant and a radius-dependent halo flattening q(Rgc). Assuming constant flattening q, the distribution of the sources is reasonably well fit by a single power law with n = 4.40 − 0.04 + 0.05 and q = 0.918 − 0.014 + 0.016 and comparably well fit by an Einasto profile with n = 9.53 − 0.28 + 0.27 , an effective radius reff = 1.07 0.10 kpc, and a halo flattening of q = 0.923 0.007. If we allow for a radius-dependent flattening q(Rgc), we find evidence for a distinct flattening of q ∼ 0.8 of the inner halo at ∼25 kpc. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results (e.g., Watkins et al.; Iorio et al.). We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after masking all known overdensities.
We analyze the luminosity function of the globular clusters (GCs) belonging to the early-type galaxies observed in the ACS Virgo Cluster Survey. We have obtained maximum likelihood estimates for a ...Gaussian representation of the globular cluster luminosity function (GCLF) for 89 galaxies. We have also fit the luminosity functions with an "evolved Schechter function", which is meant to reflect the preferential depletion of low-mass GCs, primarily by evaporation due to two-body relaxation, from an Initial Schechter mass function similar to that of young massive clusters in local starbursts and mergers. We find a highly significant trend of the GCLF dispersion sigma with galaxy luminosity, in the sense that the GC systems in smaller galaxies have narrower luminosity functions. The GCLF dispersions of our Galaxy and M31 are quantitatively in keeping with this trend, and thus the correlation between sigma and galaxy luminosity would seem more fundamental than older notions that the GCLF dispersion depends on Hubble type. We show that this narrowing of the GCLF in a Gaussian description is driven by a steepening of the cluster mass function above the classic turnover mass, as one moves to lower luminosity host galaxies. In a Schechter function description, this is reflected by a steady decrease in the value of the exponential cutoff mass scale. We argue that this behavior at the high-mass end of the GC mass function is most likely a consequence of systematic variations of the initial cluster mass function rather than long-term dynamical evolution. The GCLF turnover mass M sub(TO) is roughly constant, at M sub(to) unk (2.2 plus or minus 0.4) x 10 super(5) M \m=o.\ in bright galaxies, but it decreases slightly (by similar to 35% on average, with significant scatter) in dwarf galaxies with unk unk - 18. It could be Important to allow for this effect when using the GCLF as a distance indicator. We show that part, although perhaps not all, of the variation could arise from the shorter dynamical friction timescales in less massive galaxies. We probe the variation of the GCLF to projected galactocentric radii of 20-35 kpc in the Virgo giants M49 and M87, finding that the turnover point is essentially constant over these spatial scales. Our fits of evolved Schechter functions imply average dynamical mass losses ( Delta ) over a Hubble time that vary more than M sub(TO), and systematically but nonmonotonically as a function of galaxy luminosity. If the initial GC mass distributions rose steeply toward low masses as we assume, then these iosses fall in the range super(2) super(x) unk unk Delta < unk per GC for all of our galaxies. The trends in Delta are broadly consistent with observed, small variations of the mean GC half-light radius in ACSVCS galaxies, and with rough estimates of the expected scaling of average evaporation rates (galaxy densities) versus total luminosity. We agree with previous suggestions that if the full GCLF is to be understood in more detail, especially alongside other properties of GC systems, the next generation of GCLF models will have to include self-consistent treatments of dynamical evolution inside time-dependent galaxy potentials.
We have measured half-light radii, r sub(h), for thousands of globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC ...4697. An analysis of the dependencies of the measured half-light radii on both the properties of the GCs themselves and their host galaxies reveals that, in analogy with GCs in the Galaxy but in a milder fashion, the average half-light radius increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average half-light radius decreases with the host galaxy color. We also show that there is no evidence for a variation of r sub(h) with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average r sub(h) depends on the color of GCs, with red GCs being 617% smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed by Jordan. We discuss these findings in light of two simple pictures for the origin of the r sub(h) of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies on galaxy color, galactocentric radius, and underlying surface brightness, we show that the average GC half-light radii <r sub(h)> can be successfully used as a standard ruler for distance estimation. We outline the methodology, provide a calibration for its use, and discuss the prospects for this distance estimator with future observing facilities. We find <r sub(h)> = 2.7 c 0.35 pc for GCs with (g - z) = 1.2 mag in a galaxy with color (g - z) sub(gal) = 1.5 mag and at an underlying surface z-band brightness of k sub(z) = 21 mag arcsec super(-2). Using this technique, we place an upper limit of 3.4 Mpc on the 1s line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the r sub(h) distribution for our sample galaxies and provide an analytic expression that successfully describes this distribution.
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